Is there a simple, elegant way to define singletons?

Being relatively new to Python I’m not sure what the most common idiom is, but the simplest thing I can think of is just using a module instead of a class. What would have been instance methods on your class become just functions in the module and any data just becomes variables in the module instead of members of the class. I suspect this is the pythonic approach to solving the type of problem that people use singletons for.

If you really want a singleton class, there’s a reasonable implementation described on the first hit on Google for “Python singleton”, specifically:

class Singleton:
    __single = None
    def __init__( self ):
        if Singleton.__single:
            raise Singleton.__single
        Singleton.__single = self

That seems to do the trick.

A slightly different approach to implement the singleton in Python is the borg pattern by Alex Martelli (Google employee and Python genius).

class Borg:
    __shared_state = {}
    def __init__(self):
        self.__dict__ = self.__shared_state

So instead of forcing all instances to have the same identity, they share state.

The module approach works well. If I absolutely need a singleton I prefer the Metaclass approach.

class Singleton(type):
    def __init__(cls, name, bases, dict):
        super(Singleton, cls).__init__(name, bases, dict)
        cls.instance = None 

    def __call__(cls,*args,**kw):
        if cls.instance is None:
            cls.instance = super(Singleton, cls).__call__(*args, **kw)
        return cls.instance

class MyClass(object):
    __metaclass__ = Singleton

The one time I wrote a singleton in Python I used a class where all the member functions had the classmethod decorator.

class Foo:
    x = 1
  
    @classmethod
    def increment(cls, y=1):
        cls.x += y

The Singleton Pattern implemented with Python courtesy of ActiveState.

It looks like the trick is to put the class that’s supposed to only have one instance inside of another class.

There are also some interesting articles on the Google Testing blog, discussing why singleton are/may be bad and are an anti-pattern:

• Singletons are Pathological Liars
• Where Have All the Singletons Gone?
• Root Cause of Singletons

I’m very unsure about this, but my project uses ‘convention singletons’ (not enforced singletons), that is, if I have a class called DataController, I define this in the same module:

_data_controller = None
def GetDataController():
    global _data_controller
    if _data_controller is None:
        _data_controller = DataController()
    return _data_controller

It is not elegant, since it’s a full six lines. But all my singletons use this pattern, and it’s at least very explicit (which is pythonic).

You can override the __new__ method like this:

class Singleton(object):
    _instance = None
    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = super(Singleton, cls).__new__(
                                cls, *args, **kwargs)
        return cls._instance


if __name__ == '__main__':
    s1 = Singleton()
    s2 = Singleton()
    if (id(s1) == id(s2)):
        print "Same"
    else:
        print "Different"

class Singleton(object[,...]):

    staticVar1 = None
    staticVar2 = None

    def __init__(self):
        if self.__class__.staticVar1==None :
            # create class instance variable for instantiation of class
            # assign class instance variable values to class static variables
        else:
            # assign class static variable values to class instance variables

See this implementation from PEP318, implementing the singleton pattern with a decorator:

def singleton(cls):
    instances = {}
    def getinstance():
        if cls not in instances:
            instances[cls] = cls()
        return instances[cls]
    return getinstance

@singleton
class MyClass:
    ...

I think that forcing a class or an instance to be a singleton is overkill. Personally, I like to define a normal instantiable class, a semi-private reference, and a simple factory function.

class NothingSpecial:
    pass

_the_one_and_only = None

def TheOneAndOnly():
    global _the_one_and_only
    if not _the_one_and_only:
        _the_one_and_only = NothingSpecial()
    return _the_one_and_only

Or if there is no issue with instantiating when the module is first imported:

class NothingSpecial:
    pass

THE_ONE_AND_ONLY = NothingSpecial()

That way you can write tests against fresh instances without side effects, and there is no need for sprinkling the module with global statements, and if needed you can derive variants in the future.

Here’s my own implementation of singletons. All you have to do is decorate the class; to get the singleton, you then have to use the Instance method. Here’s an example:

@Singleton
class Foo:
   def __init__(self):
       print 'Foo created'

f = Foo() # Error, this isn't how you get the instance of a singleton

f = Foo.instance() # Good. Being explicit is in line with the Python Zen
g = Foo.instance() # Returns already created instance

print f is g # True

And here’s the code:

class Singleton:
    """
    A non-thread-safe helper class to ease implementing singletons.
    This should be used as a decorator -- not a metaclass -- to the
    class that should be a singleton.

    The decorated class can define one `__init__` function that
    takes only the `self` argument. Also, the decorated class cannot be
    inherited from. Other than that, there are no restrictions that apply
    to the decorated class.

    To get the singleton instance, use the `instance` method. Trying
    to use `__call__` will result in a `TypeError` being raised.

    """

    def __init__(self, decorated):
        self._decorated = decorated

    def instance(self):
        """
        Returns the singleton instance. Upon its first call, it creates a
        new instance of the decorated class and calls its `__init__` method.
        On all subsequent calls, the already created instance is returned.

        """
        try:
            return self._instance
        except AttributeError:
            self._instance = self._decorated()
            return self._instance

    def __call__(self):
        raise TypeError('Singletons must be accessed through `instance()`.')

    def __instancecheck__(self, inst):
        return isinstance(inst, self._decorated)

In cases where you don’t want the metaclass-based solution above, and you don’t like the simple function decorator-based approach (e.g. because in that case static methods on the singleton class won’t work), this compromise works:

class singleton(object):
  """Singleton decorator."""

  def __init__(self, cls):
      self.__dict__['cls'] = cls

  instances = {}

  def __call__(self):
      if self.cls not in self.instances:
          self.instances[self.cls] = self.cls()
      return self.instances[self.cls]

  def __getattr__(self, attr):
      return getattr(self.__dict__['cls'], attr)

  def __setattr__(self, attr, value):
      return setattr(self.__dict__['cls'], attr, value)

My simple solution which is based on the default value of function parameters.

def getSystemContext(contextObjList=[]):
    if len( contextObjList ) == 0:
        contextObjList.append( Context() )
        pass
    return contextObjList[0]

class Context(object):
    # Anything you want here

Creating a singleton decorator (aka an annotation) is an elegant way if you want to decorate (annotate) classes going forward. Then you just put @singleton before your class definition.

def singleton(cls):
    instances = {}
    def getinstance():
        if cls not in instances:
            instances[cls] = cls()
        return instances[cls]
    return getinstance

@singleton
class MyClass:
    ...

OK, singleton could be good or evil, I know. This is my implementation, and I simply extend a classic approach to introduce a cache inside and produce many instances of a different type or, many instances of same type, but with different arguments.

I called it Singleton_group, because it groups similar instances together and prevent that an object of the same class, with same arguments, could be created:

# Peppelinux's cached singleton
class Singleton_group(object):
    __instances_args_dict = {}
    def __new__(cls, *args, **kwargs):
        if not cls.__instances_args_dict.get((cls.__name__, args, str(kwargs))):
            cls.__instances_args_dict[(cls.__name__, args, str(kwargs))] = super(Singleton_group, cls).__new__(cls, *args, **kwargs)
        return cls.__instances_args_dict.get((cls.__name__, args, str(kwargs)))


# It's a dummy real world use example:
class test(Singleton_group):
    def __init__(self, salute):
        self.salute = salute

a = test('bye')
b = test('hi')
c = test('bye')
d = test('hi')
e = test('goodbye')
f = test('goodbye')

id(a)
3070148780L

id(b)
3070148908L

id(c)
3070148780L

b == d
True


b._Singleton_group__instances_args_dict

{('test', ('bye',), '{}'): <__main__.test object at 0xb6fec0ac>,
 ('test', ('goodbye',), '{}'): <__main__.test object at 0xb6fec32c>,
 ('test', ('hi',), '{}'): <__main__.test object at 0xb6fec12c>}

Every object carries the singleton cache… This could be evil, but it works great for some 🙂

The Python documentation does cover this:

class Singleton(object):
    def __new__(cls, *args, **kwds):
        it = cls.__dict__.get("__it__")
        if it is not None:
            return it
        cls.__it__ = it = object.__new__(cls)
        it.init(*args, **kwds)
        return it
    def init(self, *args, **kwds):
        pass

I would probably rewrite it to look more like this:

class Singleton(object):
    """Use to create a singleton"""
    def __new__(cls, *args, **kwds):
        """
        >>> s = Singleton()
        >>> p = Singleton()
        >>> id(s) == id(p)
        True
        """
        it_id = "__it__"
        # getattr will dip into base classes, so __dict__ must be used
        it = cls.__dict__.get(it_id, None)
        if it is not None:
            return it
        it = object.__new__(cls)
        setattr(cls, it_id, it)
        it.init(*args, **kwds)
        return it

    def init(self, *args, **kwds):
        pass


class A(Singleton):
    pass


class B(Singleton):
    pass


class C(A):
    pass


assert A() is A()
assert B() is B()
assert C() is C()
assert A() is not B()
assert C() is not B()
assert C() is not A()

It should be relatively clean to extend this:

class Bus(Singleton):
    def init(self, label=None, *args, **kwds):
        self.label = label
        self.channels = [Channel("system"), Channel("app")]
        ...

class Singeltone(type):
    instances = dict()

    def __call__(cls, *args, **kwargs):
        if cls.__name__ not in Singeltone.instances:            
            Singeltone.instances[cls.__name__] = type.__call__(cls, *args, **kwargs)
        return Singeltone.instances[cls.__name__]


class Test(object):
    __metaclass__ = Singeltone


inst0 = Test()
inst1 = Test()
print(id(inst1) == id(inst0))

As the accepted answer says, the most idiomatic way is to just use a module.

With that in mind, here’s a proof of concept:

def singleton(cls):
    obj = cls()
    # Always return the same object
    cls.__new__ = staticmethod(lambda cls: obj)
    # Disable __init__
    try:
        del cls.__init__
    except AttributeError:
        pass
    return cls

See the Python data model for more details on __new__.

Example:

@singleton
class Duck(object):
    pass

if Duck() is Duck():
    print "It works!"
else:
    print "It doesn't work!"

Notes:

1. You have to use new-style classes (derive from object) for this.

2. The singleton is initialized when it is defined, rather than the first time it’s used.

3. This is just a toy example. I’ve never actually used this in production code, and don’t plan to.

Singleton’s half brother

I completely agree with staale and I leave here a sample of creating a singleton half brother:

class void:pass
a = void();
a.__class__ = Singleton

a will report now as being of the same class as singleton even if it does not look like it. So singletons using complicated classes end up depending on we don’t mess much with them.

Being so, we can have the same effect and use simpler things like a variable or a module. Still, if we want use classes for clarity and because in Python a class is an object, so we already have the object (not and instance, but it will do just like).

class Singleton:
    def __new__(cls): raise AssertionError # Singletons can't have instances

There we have a nice assertion error if we try to create an instance, and we can store on derivations static members and make changes to them at runtime (I love Python). This object is as good as other about half brothers (you still can create them if you wish), however it will tend to run faster due to simplicity.